The present invention relates to a high-voltage variable resistor, suitable for use as a variable voltage divider for feeding a high voltage of several kilovolts to respectively focusing electrode in a cathode-ray tube of, for example, a color television receiver, and more particularly to a high voltage variable resistor with improved central slider contact construction.
A conventional high-voltage variable resistor used as a variable voltage divider, is fabricated on a rectangular insulating substrate 11 of alumina porcelain. The variable resistor comprises a film resistor 12 in the form of a circular arc which forms a variable resistance unit including film resistors 13 and 14 connected to both ends of film resistor 12. Together a stationary resistance unit is formed. A central electrode 15 is surrounded by the film resistor 12 and a slider (not shown) mounted on the flange at an end of rotary shaft slides across the film resistor 12 and the central electrode 15.
A terminal electrode 16 connected to one end of film resistor 13 is connected to a high voltage output of a fly-back transformer or the like and a terminal electrode 17 connected to one end of film resistor 14 is connected to a reference potential, such as ground. A terminal electrode 19 at the end of a film resistor 18 is connected to the central electrode 15 and forms a protective resistance adapted to conduct an attenuated high voltage from the voltage applied to the electrode 16. The film resistors 12, 13, 14 and 18 are formed of; cermet resistors each comprised of a mixture of electric conductive material, selected from the Ru group, Ag-Pd group or Rh group; glass frit for a binder or resistance value adjusting agent; and a filler, such as Al.sub.2 O.sub.3, TiO.sub.2 or Bi.sub.2 O.sub.3, for adjusting the resistance value or resistance-temperature characteristic. The central electrode 15 and terminal electrodes 16, 17 and 19 comprise a conductive material, such As Ag, Ag-Pd group, or Ag-Pt group and, as necessary, a mixture thereof with glass frit. The content of glass frit of these components usually is about 10 to 40 percent by weight in the film resistor and about 0 to 10 percent by weight in the electrode.
The central electrode 15 of conventional high voltage resistors is formed of Ag group, whereby the problem arises that the electrode 15 is subject to wear at its surface due to sliding motion of the slider. Oxidization at the surface after long use also increases contact resistance to the slider and creates the migration phenomenon which promotes a corona discharge between the electrode and the high voltage portion of the film resistor. To overcome these problems, it has been proposed to form the central electrode 15 of a material of which the film resistor 18 is made.
A central electrode constructed of film resistor, however, has a lower adhesive strength to the insulating substrate of alumina porcelain as compared to an electrode made with the Ag group material. Also, the film resistor is rich in glass constituent and is more fragile. When it is subjected to a frictional force or the press-contact force of slider it is likely to develop cracks after prolonged use. The cracks created at the center of the film resistor which forms the central electrode extends to the periphery thereof and is likely to generate corona discharges. Also, when the central electrode is brought into contact with the terminal of a resistance meter in the inspection process or the like, the contact resistance is larger and results in inaccurate measurement of resistance.